very low frequency (VLF)
The International Telecommunication Union(ITU) has classified all frequencies. The individual frequency ranges always include a power of ten starting and ending with a "3".
Very Low Frequency (VLF) is a frequency band between 3 kHz and 30 kHz. This frequency range includes parts of the audio range and ultrasound, which starts at 20 kHz. The waves in this range are referred to as longitudinal waves and have wavelengths from 100 km to 10 km.
For this frequency range, the ITU uses the band designation 4. The band number (N) and the frequency range have a mathematical relationship of 0.3 x 10expN to 3 x 10expN. The expN of band number 4 is 10,000, multiplied by 0.3 results in a lower frequency of 3 kHz.